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FDP Forum / Amp Mods, Repairs, and Projects / further musicaster bass amp advice


Austin TX

2nd most self-referential tagline ever.
Feb 24th, 2019 09:43 PM   Edit   Profile  

Hey team,
I've already subjected my 6AQ5-equipped MMB amp to the usual, recommended mods:
--yank the .0047 input cap
--replace the .01 coupling cap with .02
--convert the tone control to Harvard specs

So far so good. I've also had good results by:
--putting a 5751 in the V1 slot
--using a .47 bypass cap in first gain stage
--yanking the 15k resistor in the p/i audio tranny

Remaining issues/wishes:
1) Yanking the 15k resistor gave me a MUCH bigger, juicier sound. With more noise, granted. Is this safe? Does this affect bias? At present I have compromised by using a 22k resistor.
2) It's noisier than I'd like, although with the V1 pulled it's near dead-quiet even with volume at 10. I may have forgotten to replace the 1st electrolytic filter cap, so I will try that. What else might I try? When I was trying to reduce noise in my tweed Champ clone, I had good luck with a method that involved connecting the heater wires to 100 ohm resistors, and then to ground. Will the MMB amp circuit allow a similar fix?
3) There is no difference in volume, tone or responsiveness between amp volume at 5 and at 10. There's a crisp, raunchy sweet spot between 2.5 and 4.5 but I'd like more usable range. Given that I always keep my tone dimed, might there be value to turning the tone control into a volume control for the second gain stage?

This is a hobby. I do not gig, and I have only the mildest fantasy of turning this cheap silver box into a lightweight generator of otherworldly tone. Still, I think there's room for improvement and maybe some learning. Thanks in advance.

(This message was last edited by mongoonlypawn at 09:16 PM, Feb 25th, 2019)



Feb 28th, 2019 12:39 PM   Edit   Profile  

It should be fine to run the amp without the 15k resistor. As you’ve noticed, doing so increases gain but with that comes higher background noise.

Please describe the noise, eg hum, buzz, hiss, constant sizzle.
The heater wiring looks to be single ended rather than balanced, using the chassis as a conductor. Rewiring using a twisted pair, and adding balancing resistors (eg 100ohm) may help to reduce 60Hz hum and buzz.

Note that fully bypassing the input stage cathode is good practice in reducing coupling between its heater and cathode.

The schematic shows that the vol control is linear, which aligns with the issue described. Changing it to a log / audio type will fix it. Increasing its value to 500k or 1M will increase the max available gain a little.

Also consider re-arranging the input circuit to be like a regular Fender, ie with a 1M resistor mounted across the '1' input jack.

I'm not sure that a 2nd volume control would be much use (as a master volume at least) as there is no potential for any preamp overdrive.


Austin TX

2nd most self-referential tagline ever.
Mar 5th, 2019 08:24 PM   Edit   Profile  

Thanks pdf64.

Amp on, nothing plugged into it: slight hum, vicinity of B.
Amp on, humbucker guitar plugged in: similar, but louder and higher pitch, still largely in B.
Amp on, parts Tele plugged in: even louder, with a white noise quality added. Intermittently, there is a papery cyclical glitching that happens roughly every 5 seconds. (My Tele's shielding may merit inspection.)

Since my last report I have replaced the original 20uf filter cap coming off the preamp tube with a 20uf F&T. An A/B test showed no difference in noise.

I A/B'ed results when I added the 1M resistor across input 1. On the breadboard, the resistor acted like an antenna and produced whine. The whine subsided upon actual installation, but it didn't seem to reduce the noise so I removed the resistor.

The green heater wires coming out of the power tranny hook up to pins 3 & 4 of one 6AQ5, and thence to the second power tube and the preamp tube via twisted pairs. Where should the 100 ohm balancing resistors go? I tried installing one on each heater wire between the Xformer and the first power tube, but the amp produced no sound.

I'm afraid I don't understand your note about bypassing the input stage cathode.

Again, thanks.



Mar 8th, 2019 07:50 AM   Edit   Profile  

Does the noise stop when the instrument's volume control is turned down?
If so, the noise is due to electromagnetic / electrostatic fields in the environment; not fault of the amp, though if a non-humbucking instrument is close to the amp then it will pick up hum from the amp's PT etc.

Regarding the noise repeated every few seconds, try turning off all electrical equipment in the area; eg ceiling fans, heating, freezers, wireless routers and devices, phones, bluetooth devices.
The amp will just amplify everything the instrument sends to it; as the amp's gain is increased, any unwanted noise will inevitably also get louder.

The 1M resistor at input #1 is there to protect the tube used in the input stage, and keep it working as intended, in certain circumstances; generally it shouldn't change the sound. There's good reason most every other tube amp has them.

The 100ohm heater balancing resistors are fitted so that one leg is on a heater line, the other is connected to circuit common, ie chassis 0V. One resistor for each heater line (green wire ?).
I get the impression that you fitted them in series with the heater lines, if so the resistors would stop heaters getting power.

FDP Forum / Amp Mods, Repairs, and Projects / further musicaster bass amp advice

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